LEDs are used in a wide variety of applications and are being considered for even additional applications. For example, in today's day and age, it is advantageous to rapidly detect a release of potentially harmful biological agents, such as might occur as an act of war or terrorism, or as an accidental release. A number of government and other agency programs have addressed such detection, including the United States Department of Defense (DoD) Biological Integrated Detection System (BIDS) and the Joint Biological Point Detection System (JBPDS) program and the United States Postal Service (USPS) BioDetection System (BDS).
A current mechanism for very rapid, non-specific detection of possible biological threats is to measure ultra-violet (UV) excited fluorescence of individual airborne particles. Several sensors based on fluorescence sensing have been deployed, including the UV Active Protection Sensor (UV-APS) in the BIDS and the Biological Agent Warning Sensor (BAWS) in the JBPDS system. One drawback of using these sensors deals with the large costs involved. For instance, these sensors typically rely on relatively expensive YAG lasers and photomultiplier tube (PMT) optical detectors, and may cost $40,000 to $100,000.
Recent developments of UV LEDs that could potentially replace the more costly lasers as the excitation source in a fluorescence sensor permits the design of next-generation low cost fluorescence sensors. Regardless of the laser source, it is advantageous to match the wavelength of the excitation source to absorption characteristics of fluorophores in biological threat agents. Multiple excitation and/or fluorescence emission wavelength bands can be advantageously used to better discriminate targets for detection by permitting discrimination between and identification of specific chemical compounds or compositions.
One type of UV LED being considered for use in the design of a fluorescence sensor operates at a wavelength of 280 nanometers. This wavelength makes it a candidate for replacing a YAG laser, since it is near the peak excitation for tryptophan—one target of interest. However, one disadvantage of using current UV LEDs operating at a wavelength of 280 nanometers is that the LED output power in current devices is about a factor of 10 or more lower than what is desired for good sensitivity of the sensor.
In order to increase the output power of the LED, more input electrical current has to be supplied to the LED junction. However, inputting more electrical current to the LED junction results in an increase in the thermal load and a consequent reduction in the lifetime of the LED. Increasing the emitting area of the diode of an LED may help to distribute the thermal load, but typically results in current depletion in regions of the emitting area that lead to unacceptable non-uniformity in the projected spot distribution.
Currently, LEDs operating at a wavelength of 280 nm are available in 2×2 arrays on a single chip. The geometry of these LEDs is typically such that the emitting areas are 75 μm by 200 μm and are separated by 350 μm in a square grid arrangement. In principle, larger arrays of LEDs can be manufactured. The shape and dimensions of the emitting area are typically designed to maximize the total output power while maintaining an approximately spatially uniform emission pattern. Since these are competing performance parameters, there is always a trade-off of power for uniformity.
In view of the foregoing, there is a need for an UV-LED that can replace costly lasers as an excitation source in a fluorescence sensor or in other applications that provides suitable total power while reducing or eliminating tradeoffs between uniformity and emitting area size, thereby providing a sufficiently uniform irradiance in an image plane for the respective applications.